1. Field of the Invention
The present invention relates to an optical isolator and, more particularly, to an optical isolator which is not dependent on plane of polarization and utilizes polarization splitters made of birefringent crystal.
2. Prior Art
Optical isolators are usually constructed from a combination of a plurality of bulk optical parts such as polarizers, lenses, Faraday rotators, birefringent elements, half wave plates, etc. and accordingly, are accompanied by a number of problems including requirements of a number of parts, large overall size and troublesome and time-consuming adjustment between the parts. To cope with these problems, a proposal was made to minimize the size of the optical isolator by directly incorporating an optical isolator in an optical fiber system.
Japanese Patent B-3-22962 discloses an optical isolator having one integral optical fiber which is cut at one or more axial positions to form one or more grooves and a multi-layer dielectric metal layer polarizer, Faraday rotator, etc. are inserted in the groove or grooves.
The present invention utilizes this type of technique but the one disclosed in this prior art is of a type which is dependent on plane of polarization.
U.S. Pat. No. 4,978,189 and Japanese Patent A-3-196115 disclose optical isolators which are not dependent on the plane of polarization. The optical isolators disclosed in these references comprise an optical waveguide having a polarization splitter inserted therein to split the incoming light into two polarized lights, which are then guided by a pair of optical waveguides to respective Faraday rotators and half wave plates. The light beams are then recombined by a recombinant polarization splitter. However, with this device, it is difficult to manufacture polarization splitters for the splitting and recombination.
Japanese Patent A-4-307512 and Japanese Patent A-4-349421 disclose optical isolators which do not depend on the plane of polarization and which comprise an optical fiber embedded in a substrate plate, one oblique slit formed in the substrate plate by obliquely cutting the plate and the optical fiber together, and an optical isolator element (consisting of three beam splitters and a Faraday rotator; or two beam splitters, a half wave plate and a Faraday rotator) inserted in the slit. The angle of the end surface of the optical fiber and the thickness of the elements are so selected that the angle of inclination of the light beam exiting from the fiber's oblique end is compensated for by the elements as if the light travels along a straight pass through the optical fiber and the return lights reflected by end surfaces of the elements are prevented from returning to the input side owing to the oblique cut. According to this technique, the isolator is easily assembled without any adjustment but the total thickness of the optical isolator is made thick due to the fact that all of the elements are integral and has a drawback of large diffraction loss.
Thus, the techniques disclosed in these Japanese Patent A-4-307512 and Japanese Patent A-4-349421 have the drawback of large optical loss.
On the other hand, proposed as splitters which maybe used in such an optical isolator is an optical switch disclosed in Japanese Patent A-2-18525, for example. The optical switch of this type comprises four terminal optical waveguides having two input terminals and two outputs terminals formed on a substrate plate, a polarized light splitter film inserted in a slit formed at a crossing point of two input-side waveguides, a polarized light splitter film (for recombining the polarized lights) inserted in a slit formed at a crossing point of two output-side waveguides, and a magneto-optical element (Faraday rotating element) and a half wave plate both inserted in a single slit formed between the two optical splitter films. Any one of the input terminals can be connected to any one of the output terminals by reversing a magnetic field applied to the magneto-optical element.
On the other hand, U.S. Pat. No. 4,978,189 discloses an optical switch by controlling the effective refractive index at the crossing points in place of using polarized light splitter films disclosed in Japanese A-2-18525.
However, these polarization splitters disclosed in these literatures require crossing points of the optical waveguides which are difficult to manufacture with a sufficient precision.
A principal object of the present invention is to provide an optical isolator which is low in loss and easy to assemble with no adjustment, and in which the two polarized lights have the same length of path and thus the polarization mode dispersion (phase difference introduced depending on the polarization) is low.
Another object of the present invention is to provide an optical isolator having a large return loss and to provide an polarization splitter therefor.
A further object of the present invention is to provide an optical polarization splitter which has no crossing point of optical waveguides.